GLOBAL POPULATION-STRUCTURE OF THE SWORDFISH (XIPHIAS-GLADIUS L) AS REVEALED BY ANALYSIS OF THE MITOCHONDRIAL-DNA CONTROL REGION

The global population structure of the swordfish (Xiphias gladius L.) was examined by analyzing the DNA sequence variation contained within the hypervariable left domain of the mitochondrial control region of 2 47 individuals. A total of 330 base pairs (bp) of sequence from 112 in dividuals collected in the Pacific (n = 26), the Atlantic (n = 47) and the Mediterranean (n = 39) revealed a total of 69 haplotypes. Extreme ly high values of haplotypic diversity characterized all samples and a ll haplotypes occurred at low frequencies. The topology of a neighbor- joining tree was used to establish phylogenetic relationships among th e mtDNA lineages. The genotypes could be assigned to 2 highly divergen t clades, namely clade I and clade II (net nucleotide difference 3.9%) . Furthermore, clade I could be divided into 2 groups designated alpha and beta. The presence of phylogenetically informative differences in the DNA sequence allowed the design of a Restriction Fragment Length Polymorphism (RFLP) assay which could discriminate members of the 3 su bgroups. Using this phylogenetically based RFLP assay, an additional 1 35 individuals from the Atlantic and the Mediterranean were classified as members of the alpha, beta or theta (clade II) subgroups. The inte r-oceanic heterogeneity of swordfish haplotype frequencies was tested on the combined data sets using both Monte Carlo distributions of chi- square values and G-tests. The results indicated that haplotype freque ncies in the samples from the Pacific Ocean, the North Atlantic Ocean, the South Atlantic Ocean and the Mediterranean Sea were significantly different. Comparisons of haplotype frequencies between samples from the East and the West North Atlantic corroborate previous studies whic h indicated that extensive mixing has taken place between these region s and is consistent with a hypothesis of no difference. However, this aspect of the analysis is limited by both small sample sizes and the p ossibility of populations mixing on the feeding grounds.